The present invention is directed to heat transfer tubes used in the radiant section of a fired heater, and more particularly to radiant tubes provided with smooth surfaces at relatively high flux areas on the outside of the tube and extended surfaces on relatively low flux areas on the outside of the tube.
Combustion equipment is generally operated in chemical plants, petrochemical plants and refineries. The equipment may include industrial heaters, furnaces or plant boilers. This equipment is generally designed with bare or smooth-walled tubes. Use of bare tubes in radiant sections usually exposes the front half of the tube to direct flame radiation, while limiting the exposure of the rear half or dark side of the tube to reflected radiation. The difference in exposure is sufficient to cause the opposing sides of the tube to expand at different rates. As a result of the thermal stresses, the tubes will bow towards the direct flame radiation. To prevent tube stress the tube from exceeding design temperatures, the tube temperature must be maintained and regulated within specified safe temperature limits for the particular tube material.
The heat flux distribution around the circumference of a conventionally fired or radiating plane is on one side of the tube and a refractory wall is on the other. The front half of the tube surface faces the flame (point A) and receives a higher heat flux as compared to the rear half facing the refractory wall (point B). Point A receives heat flux only from direct flame radiation, while point B, facing the refractory wall, receives only reflected radiation coming from the refractory wall. Points between point A and point B receive varying amounts of both direct and reflected radiation, depending upon their location along the tube.
The standard distance between tubes is two tube diameters from center-to-center for most operations in the chemical and petrochemical industries, as shown in FIG. 2. The heat flux distribution in FIG. 1 is based on this configuration. For the purposes of an illustration using fluxes typical in a conventional fired heater, where the highest heat flux at point A is 18000 Btu/hr-ft2, the diametrically opposed counterpart (point B) receives only 6000 Btu/hr-ft2. The rear half of the tube transfers only 24% of the total heat absorbed by the tube; this includes both the direct and reflected radiation, as seen in FIG. 3. The average flux for the tube amounts to 10,000 Btu/hr-ft2.
More than 85% of the heaters in the industry have such a large flux differential between the front and the rear side of the tube, as this illustration depicts. A significant compromise is made on the overall heat-receiving capacity of the tube in order to keep the flame-front side (point A) within safe working temperatures.
To make the heat flux distribution in the tube more uniform, one approach of the furnace designers has been to increase the center-to-center tube spacing requirements from 2 to 3 tube diameters. This design increases the flux at point B of the tube from 6,000 Btu/hr-ft2 to 9,000 Btu/hr-ft2 as shown in FIGS. 4A and 4B. The increased spacing has the beneficial result of increasing the heat-receiving capacity of the rear half of the tube for the 3D-spaced tubes, while heat flux distribution on the front half of the tube is generally the same as for the 2D-spaced tubes. This results in an increase of the average heat flux to 12,000 Btu/hr-ft2 for the entire tube. However, the drawback of this solution is apparent. With an increase in tube spacing there is a corresponding increase in the size of the heater. This increases the cost and space requirements for the heater.
Another prior art approach improves the heat flux distribution by placing radiating flames on opposing sides of the tubes in a so-called xe2x80x9cdouble-firedxe2x80x9d design. A comparison is shown between one radiating flame (A) and two radiating flames (B) in FIGS. 5A and 5B, respectively. This design is commonly used in chemical processes that mandate a more uniform heat flux distribution, such as, for example, in delayed cokers, high-pressure hydrotreaters, ethylene furnaces, and the like. In a double-fired system, the front (point A) and rear (point B) portions of the tube have the same heat flux rate due to direct flame radiation, and the points at the margins between the front and rear receive relatively less direct flame radiation. The corresponding distribution of the heat flux, for the illustrative example, is 18,000 Btu/hr-ft2 for the front and the rear locations, 13,500 Btu/hr-ft2 at the margins between the front and rear faces, i.e. the middle area of the tube (point M at the 90 and 270 degree positions), resulting in an average flux of 15,000 Btu/hr-ft2. The double-fired design brings with it the disadvantage that the heater has to be much larger, as much as twice the size as a single-fired unit, and correspondingly more expensive.
The present state of technology for heaters with a standard spacing of 2 tube-diameters will have a relative flux ratio of 1 to 1.8 between the average flux and the maximum flux, whereas a heater with a 3 tube-diameter spacing will have a relative flux ratio of 1 to 1.5, as shown in API Standard 530, Calculation of Heater-Tube Thickness in Petroleum Refineries, American Petroleum Institute (1988), FIG. C-1 Ratio of Maximum Local to Average Heat Flux Curves, page 103).
The 3 tube-diameter design is less common in the industry and the vessel must be significantly larger than a 2 tube-diameter design. The average to maximum flux ratio of the double-fired tubes is significantly lower at 1 to 1.2, but is a more costly alternative of the three designs for an industrial plant.
The present invention utilizes extended surfaces, such as studs and fins, on the low-flux area(s) of radiant tubes to more uniformly distribute heat flux. In the single-fired furnace design, for example, the invention increases the overall heat transfer of the tube by increasing heat flux rate for the backside of the tube and thus decreases the temperature differential between the opposing tube sides.
In one aspect, the invention provides a tube for use in a fired furnace wherein the tube is disposed longitudinally between a flame and a refractory wall. The tube has a central longitudinal bore for the passage therethrough of a fluid to be heated, an imperforate outside diameter having a radiant side for exposure to radiation from the flame and a dark side essentially free of direct exposure to the flame, and a plurality of extended surfaces positioned on at least a part of the dark side of the outside diameter effective to increase heat flux of the dark side. The radiant side of the outside diameter is essentially free of extended surfaces, excluding margins thereof adjacent the dark side which can optionally be provided with extended surfaces. The extended surfaces are preferably studs or fins welded to the dark side of the outside diameter.
In another aspect the invention provides a fired furnace comprising a plurality of the tubes as described above, wherein the tubes are disposed in the furnace between a flame and a refractory wall with the radiant side of the outside diameter facing the flame and the dark side facing the refractory wall.
In a further aspect, the invention also provides an improvement in a fired furnace comprising a plurality of tubes disposed between a flame and a refractory wall, each tube including a central longitudinal bore for the passage therethrough of a fluid to be heated, an outside diameter having a radiant side for exposure to radiation from the flame and a dark side essentially free of direct exposure to the flame. The improvement comprises a plurality of extended surfaces disposed on the dark side of the outside diameter and a smooth surface substantially free from extended surfaces disposed on the radiant side of the tubes, except optionally at the margins thereof adjacent to the dark side which can optionally be provided with extended surfaces.
A still further aspect of the invention is the provision of a method for improving the heat transfer in a fired furnace comprising a plurality of tubes disposed between a flame and a refractory wall, each tube having a smooth outside diameter essentially free of extended surfaces. The method includes the steps of removing and replacing one or more of the tubes in the furnace with tubes including a central longitudinal bore for the passage therethrough of a fluid to be heated, an outside diameter having a radiant side for exposure to radiation from the flame and a dark side essentially free of direct exposure to the flame, and a plurality of extended surfaces positioned on at least a part of the dark side of the outside diameter effective to increase heat flux of the dark side, wherein the radiant side of the outside diameter, optionally excluding margins thereof adjacent the dark side, is essentially free of extended surfaces.
A related aspect of the invention is the provision of a method for improving the heat transfer in a fired furnace comprising a plurality of tubes disposed between a flame and a refractory wall, each tube having a smooth outside diameter essentially free of extended surfaces. The method includes the steps of mapping the heat flux on the exterior surface of the smooth tubes in the furnace, determining the relatively low flux areas of the tubes, removing the smooth tubes from the furnace, installing extended surface structures on the exterior surface of replacement tubes at the low flux areas determined from the mapping step, and installing the replacement tubes in the furnace. The replacement tubes have a central longitudinal bore for the passage therethrough of a fluid to be heated, an outside diameter having a radiant side for exposure to radiation from the flame and a dark side essentially free of direct exposure to the flame, and extended surfaces comprising a plurality of the extended surface structures positioned on at least a part of the dark side of the outside diameter effective to increase heat flux of the dark side, wherein the radiant side of the outside diameter, optionally excluding margins thereof adjacent the dark side, is essentially free of extended surfaces. The extended surfaces can have an area that varies proportionally to the difference between the maximum heat flux of the tube and the mapped heat flux in the vicinity of the extended surface structure. The area of the extended surfaces can be varied by varying the proximity of the extended surface structures to adjacent extended surface structures, or by varying the area of the extended surface structures relative to adjacent extended surface structures.
The extended surfaces on the tubes described above can have a larger area in a central region of the dark side relative to the area of the extended surfaces adjacent to margins between the dark and radiant sides. Alternatively or additionally, the extended surfaces can have an area that varies generally inversely proportional to heat flux. For example, studs measuring xc2xd-in. diameter by xc2xd-in. High can be welded to the exterior of the tube at the point closest to the refractory wall, while studs measuring xc2xc-in. diameter by xc2xc-in. tall can be welded to the exterior of the tube at the margins between the dark and radiant sides, and the studs between these can vary gradually from xc2xc-in. at the margins to xc2xd-in. at the center of the dark side.
In another aspect the invention provides a tube for use in a fired furnace wherein the tube is disposed longitudinally between flames on either side thereof. The tube includes a central longitudinal bore for the passage therethrough of a fluid to be heated, an outside diameter having opposing radiant sides for exposure to radiation from a respective flame, opposing relatively low-flux margins between the radiant sides, and a plurality of extended surfaces positioned in the margins on the outside diameter of the tube effective to increase heat flux at the margins and reduce the ratio of maximum to average heat flux. The radiant sides of the outside diameter between the margins is essentially free of extended surfaces.